The present invention relates to an optical external modulation element.
A semiconductor laser has a small size, high-efficiency and high-reliability, and it has already been put to use as the light source for optical fiber communications. Another noteworthy feature of the semiconductor laser is to permit direct modulation, but high-speed direct modulation causes an increase in the spectral width of the semiconductor laser, constituting a serious obstacle to long-distance, large-capacity optical fiber communication. In particular, an ordinary semiconductor laser utilizing the cleavage plane oscillates at a plural wavelengths during high-speed modulation, and hence it has been employed only in the 1.3 .mu.m wavelength band in which no wavelength dispersion of the optical fiber occurs but loss is relatively large. On the other hand, a distributed feedback semiconductor laser, which performs a single-wavelength operation during high-speed modulation as well, is insusceptible to the influence of the wavelength dispersion, and it is now being developed intensively for use in the 1.5 .mu.m band in which loss is small. When the modulation rate exceeds 1 giga bit/sec (Gb/S), however, even if it oscillates at a single wavelength, the spectral width increase owing to a change in the injected carrier density poses a problem that the influence of dispersion is nonnegligible.
Chirping (variation) of the oscillation wavelength or the resulting spread of the spectral width during high-speed modulation is unavoidable as long as the semiconductor laser is subjected to the direct modulation. Then the external modulation technique is considered promising which allows the semiconductor laser to operate at a single wavelength in the steady state and modulates the output light outside the resonator. With the external modulation technique, since the static spectral width (.ltoreq.10 MHz) increases by the modulation band (.about.GHz) alone, the increase in the spectral width can be reduced down to about 1/10 that (1 to 3 .ANG.) in the case of the direct modulation.
As such a conventional waveguide type external modulator, directional coupling type and Mach-Zender interference type structures utilizing a ferrodielectric material are mainly attracting attention in terms of the modulation band and the extinction ratio. However, these modulators performs intensity modulation of light by changing its phase velocity, and hence are defective in that they must be fabricated uniformly and precisely controlling the dimensions of the waveguide, that the modulation element is as long as several milimeters to several centimeters and that the insertion loss is also large.